Fusarium Wilt of Lettuce

Lettuce grown in California has a farm gate value of nearly 2 billion dollars in most years, making it one of the state’s most valuable agricultural commodities. In 1990, plants with symptoms that ranged from mild stunting to complete collapse were observed in two fields near Huron, in the San Joaquin Valley.

Diseased plants had severely rotted taproots, from which a fungus identified as Fusarium oxysporum was consistently isolated. Fusarium oxysporum is widespread in agricultural soils throughout the world and is commonly isolated from roots of healthy plants. Most strains are weak parasites that cause no visible damage to their host plant, within which their growth is limited to the root cortex. However, some strains can invade the water conducting tissue (xylem) and thus restrict the flow of water and cause wilting.

Host-specific, disease-causing strains of F. oxysporum are referred to as formae speciales (f. sp.) to distinguish them from non-pathogenic strains. The host-specific strain causing disease on lettuce is known as F. oxysporum f. sp. lactucae.

Our studies have shown that the risk of damage from Fusarium wilt is influenced by three factors: cultivar susceptibility, the abundance of pathogen inoculum and ambient temperatures during the growing season. Some leaf and romaine cultivars are highly resistant, and remain healthy under all conditions, but for most cultivars – including all head types that have been tested – disease severity will be enhanced by higher soil inoculum densities and warmer weather (1). The risk of severe disease may thus be reduced by crop rotation to allow for attrition of pathogen propagules in soil (2), and by not growing susceptible cultivars during the warmest planting windows (3). Presently we are working with plant breeders to move the determinants of resistance from certain romaine and leaf cultivars into the genetic background of a head lettuce type.

Projects in my lab include applied and basic investigations into the biology of plant pathogenic fungi, primarily species of Fusarium and Verticillium. Pitch canker disease on Monterey pine, caused by Fusarium circinatum, is currently a major focus of activity. This includes studies on the breeding biology of the pathogen, constitutive and induced disease resistance in the host, and the interactions between the fungus and its insect vectors.